/** 
 * Forward declaration of guess API.
 * @param  num   your guess
 * @return 	     -1 if num is higher than the picked number
 *			      1 if num is lower than the picked number
 *               otherwise return 0
 * int guess(int num);
 */

class Solution {
public:
    int guessNumber(int n) {
        int left=1;
        int right=n;
        while(left<right)
        {
            int mid=left+(right-left)/2;
            if(guess(mid)<=0)
            {
                right=mid;// 答案在区间 [left, mid] 中
            }
            else
            {
                left=mid+1;// 答案在区间 [left, mid] 中
            }
        }
        return left;
    }
};